Expand Minitest related instructions on `README`
Expand Minitest related instructions on `README`

* Add a Minitest specific section to the README, similar to `Test::Unit`
* Add `Capybara::Minitest::Assertions` and `require`s to all
  Minitest and Rails examples.

teamcapybara/capybara,teamcapybara/capybara,teamcapybara/capybara,jnicklas/capybara,jnicklas/capybara,jnicklas/capybara